Blinded comparison of computed tomography, ultrasound and needle methods to measure skin flap thickness for cochlear implantation

Purpose: Patient suitability for cochlear implant (CI) devices compatible with magnetic resonance imaging and CI processor configuration is dependent on their retro-auricular skin flap thickness. This is typically measured intra-operatively using a needle and therefore patients are not guaranteed their implant of choice prior to surgery. We aimed to identify an accurate method to measure skin flap thickness pre-operatively to streamline CI selection and simplify the consent process. Methods: Blinded prospective skin flap thickness measurements for patients undergoing CI surgery were recorded using pre-operative computed tomography (CT) and ultrasound (US), and intraoperative needle measurement. Results: Fifty-six adult patients (36 females, 20 males; mean age 59 years) were included. The mean flap thickness was measured highest by CT (6.9 mm, 95% CI 6.5–7.3 mm), followed by US (6.3 mm, 95% CI 5.9–6.7 mm) and lastly needle (5.5 mm, 95% CI 5.1–5.9 mm) (p < 0.0001). A strong positive correlation (p < 0.001) was noted between all three modalities: CT vs needle (r = 0.869), US vs needle (r = 0.865), and CT vs US (r = 0.849). Conclusion: Accurate, non-invasive measurement of skin flap thickness prior to CI surgery can be achieved using CT or US. We recommend the routine use of US in the outpatient clinic to minimise unnecessary radiation exposure.